Acute effects of salinity exposure on glochidia viability and host infection of the freshwater mussel Anodonta anatina (Linnaeus, 1758).

Freshwater mussels of the Unionida are globally in decline and knowledge of the sensitivity of their vulnerable life stages to stressors is crucial for their conservation. Increasing salinity, e.g., due to road deicing, has been proposed as an important stressor, but its impact on the complex interaction between mussel larvae and their obligate attachment to fish hosts remains largely unknown. This study tested the acute toxicity effects of environmentally relevant chloride concentrations on larvae of European Anodonta anatina mussel as well as the impacts on attachment rates of the larvae to their fish host. Chloride concentrations above 727 mg⋅L(-1) significantly affected glochidia viability and the 24h EC50 value was determined at 2,505 mg⋅L(-1). Successful attachment of glochidia to the host fish Phoxinus phoxinus was negatively correlated with increasing chloride concentration and became significant at concentrations > 2,909 mg⋅L(-1). Comparable responses could be observed by separately counting fin and gill attached glochidia, while gill attachment showed the highest correlation with overall attachment rates. These results indicate a potential threat from short-term elevated chloride concentrations during runoff events on sensitive life stages of freshwater mussels. Consequently, we propose additional chloride sensitivity tests on other mussel species as well as the reduction of salt peak input loading into freshwater bodies through a 3R-principle (restriction of use, retention of runoff for peak concentration avoidance and replacement by alternatives) in areas where endangered mussels occur. CAPSULE: Freshwater mussels of the Unionoida are globally in decline and knowledge on the sensitivity of the most vulnerable larval stages to salinity is crucial for their conservation.